Cream butter and sugar together in bowl of stand mixer. Add vanilla extract. Add eggs one at a time, mixing well after each addition. Pulse gluten free cooking oats until they are crushed into a fine powder. Add white rice flour and sweet rice flour and process for a minute or two. Add potato starch, baking powder and salt- pulse until well combined.
With mixer on low, add dry ingredients to the butter/egg/sugar mixture. Once combined, scrape dough out of bowl and form into a ball onto a large piece of saran wrap. Cover fully with saran wrap and refrigerate overnight.

To roll out shapes:
Cut refrigerated dough into two equal halves. Place one half onto a clean counter space dusted with sweet rice flour and place the other half back into the refrigerator. Dust a rolling pin with rice flour and roll dough into an even sized oval or circle (about 1/2 inch thick). Coat cookie cutters in flour and cut shapes out of dough. Cut and lift carefully, as the dough is more crumbly than traditional sugar cookie dough. Place cut out cookies onto parchment paper or silpat lined baking sheets. These cookies spread more than my favorite "with gluten" sugar cookie recipe, so I recommend popping the baking sheet in the freezer for about 10 minutes prior to baking.
Bake cookies at 350 degrees for about 10 minutes, or until bottoms are slightly golden.

Royal Icing Recipe:
Pasteurized egg whites from 2 large eggs
2 cups confectioner's sugar
1 tsp vanilla extract
Using the whisk attachment for a stand mixer, beat whites and vanilla on medium speed until foamy. Reduce speed to low and add in sifted confectioner's sugar. Once all of the sugar is added, increase speed to high and beat until icing forms stiff peaks (6-7 minutes).
Separate icing into as many bowls as you want colors. Color each icing as desired. Using a small round tip (wilton size 3 or 5 tip) fitted to a pastry bag, outline each cookie with the thick icing. Once all cookies are outlined, add a teaspoon of water to each bowl (and add more, if necessary) to thin the icing to "flood" the cookies. The icing should go on easily, but should not be runny.
Allow cookies to dry overnight.